Add credentials_file_path configuration parameter for file-based credential support

[reyhankoyun]

PR 1/3 for file-based credential support.

Adds an optional credentials_file_path configuration parameter and the default credentials file path constant. No runtime behavior change, the field is parsed and stored but not yet consumed.

[reyhankoyun]

The 2 partial coverage lines are ? error branches on set_default() calls, identical to every other set_default line in the same function, which also have uncovered error branches. No way to exercise these without mocking the config library internals.

[simonmarty]

Should we add a negative test case (tries to pass malformed pass to the config, fails, expect a specific error)?

[reyhankoyun]

Should we add a negative test case (tries to pass malformed pass to the config, fails, expect a specific error)?

Maybe not for this PR. The credentials_file_path field is Option<PathBuf>, any string is a valid PathBuf, so there's no malformed value that would fail at the config parsing stage. A path like /tmp/nonexistent is a valid PathBuf; it just doesn't point to an existing file, which will be handled at runtime in a follow up PR.